1. Field of the Invention
The present invention generally relates to electrical power supplies and, more particularly, to an improved variable speed alternator driven power supply which provides automatic overvoltage protection in gated rectifier phase control circuits.
2. Description of the Prior Art
Variable speed alternator driven power supplies are well known in the art, perhaps the most notorious example of which is the conventional charging system of an automobile. The automobile alternator is characterized by a wound rotor; however, in some marine applications, such as torpedo systems, it is essential to utilize an alternator having a permanent magnet rotor. In these applications, the permanent magnet rotor alternator has two important advantages over a wound rotor alternator. First, the permanent magnet rotor alternator can supply a much higher power output for a given alternator size or volume than a wound rotor alternator. Second, the permanent magnet rotor alternator output, when controlled by a gated rectifier bridge, can be easily controlled so as to produce a zero output voltage, this being a critical strategic requirement of certain load systems adapted to be energized from the output of the alternator.
The setting for my invention is the necessity of providing a power supply meeting the following specifications:
(i) a very small but high power and high frequency alternator with a permanent magnet rotor where the mechanical drive to the rotor has a substantial variation in angular velocity while the power supply is under load;
(ii) a gated rectifier circuit, such as a silicon controlled rectifier (SCR) bridge circuit, and
(iii) a phase control circuit operatively connected to the gated rectifier circuit and the alternator in a closed loop system.
Several problems need to be overcome in order to satisfy the foregoing specifications. First, due to the fact that very small, high frequency alternators exhibit output voltages which become severely distorted under load, and this distortion tends to prevent the use of the state of the art SCR control techniques. Second, permanent magnet rotor alternators produce an electrical output, the magnitude of which is a direct function of rotor speed. In fact, under "no-load" conditions, the alternator output voltage and frequency are directly proportional to the rotor speed, and this variation in alternator output voltage can cause a very undesirable corresponding variation in the output of the SCR bridge, if not regulated by closed loop control.
To solve the first problem, a reference circuit was required having an output reference voltage which was (a) synchronized to the alternator frequency and (b) was substantially free from harmonic distortion which causes jitter and/or ambiguous operation. To solve the second problem, a special closed-loop regulating system was required.
There, however, remains a third problem which can occur if feedback is lost, a condition which might be caused, for example, by a feedback amplifier failing high or low. More specifically, what can happen under this condition is that the output voltage from the SCR bridge may exceed the rated voltage of the power amplifiers, typically metal oxide silicon field effect transistors (MOSFETs), in an invertor circuit supplied by the alternator driven power supply. The result is instant MOSFET burnout and resulting inoperativeness of the circuits supplied by the invertor circuit.